There's nothing quite like the sated, happy feeling that comes with a great dinner party: The perfectly matched wines, the seasonal vegetables done just so, all that buoyantly witty conversation you'll be thinking about for days to come.

And then, just an hour or so later, you'll quietly slip off to the bathroom and, what the...!?!

Oh, right. Asparagus.

Now, at about this point, some of you are nodding (lowered) heads in recognition, while the rest of you are muttering, "What's he talking about?"

There's a reason for this. Only some of us have digestive systems that almost instantly turn asparagus into really foul smelling urine. And not everyone's nose can detect the odour, no matter how rank.

Nor do we have a lot of outside points of reference. The asparagus phenomenon is unique within the world of veggies.

"There doesn't seem to be anything else that does the same thing," says Jim Atkinson, a nutritionist and professor of animal and poultry science at the University of Guelph.

So why asparagus? What is it that turns all those lovely sprouts of spring into something truly obnoxious?

And why do only some of us end up feeling like the Wicked Witch of the West from The Wizard of Oz, whose smoky departure prompts The Good Witch of the North to remark: "Pooh, what a smell of sulphur."

As it happens, scientists have been pondering those kinds of questions for a very long time, and they still can't agree on a universal set of answers.

In a recent paper called "Food Idiosyncrasies: Beetroot and Asparagus," Steve Mitchell of the Imperial College School of Medicine in London recounts decades of sometimes conflicting theories and research.

For instance, it wasn't until the 1950s that researchers concluded that not everyone produced the smell. So, for the first time, populations started getting divided into "excretors" and "non-excretors."

As Mitchell points out, it may have been "understandable" that it took so long to figure this out.

"Those who produce the odour assume, politely, that everyone does, and those who do not produce it have no idea of the olfactory consequences," says Mitchell. "There is no reason as to why these two opposing factions should converse on this subject."

Various studies over the last half-century have suggested that perhaps half of the people in Britain are excretors, while the figures for the United States are higher and one French study of more than 100 people found they were all malodorous after eating asparagus.

So it's likely there are genetic factors at work, though no one studying the question has as yet gone to the trouble of isolating DNA. But Terry Graham, chair of human health and nutritional sciences at the University of Guelph, knows at least this genetic fact: Babe Ruth once remarked about the smell, "This is one way of identifying how closely related to Babe Ruth you are."

In other words, to be even remotely related to the Babe, you have to possess a digestive system with the right enzyme or enzymes to "catabolize" — or break down — some inherent part of asparagus and turn it into a smelly waste product.

For scientists, this raised two broad questions: What is in the asparagus? And what exactly do some of us turn it into?

On both fronts, there were a raft of variables and unknowns.

Over the years, they managed to isolate a series of sulphur compounds in post-asparagus urine, all of them with tongue-twisting names like S-methylthioacrylate and tetrahydrothiophene.

But just because they're present in a liquid doesn't necessarily mean they're still volatile enough to produce a vapour.

`Those who produce the odour assume, politely, that everyone does,

and those who do not produce it have no idea of the olfactory consequences'

Steve Mitchell

The Imperial College School of Medicine, London

So that sent scientists off to do other tests, including gas spectrometry, to figure out which sulphur compounds were the most volatile.

They found plenty of these, too, but settled on two key ones: methanethiol and demethyl sulphide.

There was, alas, a problem with this. Those two compounds are so volatile that they would have been released from the asparagus during cooking. They're also found in a raft of other vegetables.

What was needed was some component unique to asparagus that remained stable enough when cooked that it could then make its way to the digestive system more or less intact.

The answer — or at least the consensus after a half-century of research — is something scientists are calling "asparagusic acid."

That's the stuff that we, or some of us, turn into noxious fumes of sulphur.

You won't find asparagusic acid in anything else we eat, although its close relatives turn up in everything from tropical mangroves to marine worms.

Asparagusic acid, it turns out, is what young asparagus use to ward off parasites. As the plant ages, however, the concentrations decline.

We would have seen this coming, really, if anyone had first consulted Samuel Johnson's 1755 Dictionary of the English Language.

Johnson quotes a slightly earlier book by Queen Anne's physician, a Scot by the name of John Arbuthnot, who wrote that asparagus "affects the urine with a foetid smell (especially if cut when they are white) and therefore have been suspected by some physicians as not friendly to the kidneys."

Arbuthnot adds: "When they are older, and begin to ramify, they lose this quality; but then they are not so agreeable."

But one problem remains: Why doesn't everyone smell it?

"There might be some people who are odour-blind to it," says Alan Hirsch, neurological director at the Smell & Taste Treatment and Research Foundation in Chicago.

Between five and 10 per cent of people with an otherwise normal sense of smell have these kinds of blind spots, although Hirsch has never tested specifically for asparagus.

And the ability to smell does decline with age, beginning around 35.

By age 65, about half of people will experience some lowered sense of smell, which jumps to 75 per cent by the time we reach 80. "So for them, even if they have the smell in their urine, they won't smell it," says Hirsch.

But here's the thing about smells, and why that asparagus-based sulphur can hit you so quickly: The nose is the only sensory organ that's essentially hard-wired to the part of the brain that deals with emotions. Everything else — especially sight — needs some kind of translation, turning, say, a picture of a horse into the idea of a horse.

"The quickest way to change somebody's mood or behaviour is through smell," says Hirsch. "You'll smell something and immediately know if you like it or don't like it, which is totally different from all the other sensory spheres."

That could prompt some people to avoid asparagus in the first place, but this might be a mistake. You'd miss all that flavour, for one.

And there's the small matter of what asparagus says about your personality. During the past 20 years, Hirsch's group has done standard personality tests on nearly 19,000 Americans and then correlated the results with their food preferences.

Those who adore asparagus, it turns out, tend to be "dramatic, theatrical people who like to be the centre of attention," says Hirsch. They're also "lively, enthusiastic, flirtatious, provocative and seductive," with a penchant for "flights of romantic fantasy."

So maybe, in the end, that's why any gathering of asparagus lovers often turns out so memorably.